This invention relates generally to the energy-beam-driven deposition of a material onto a growth surface, and more particularly to a system for the energy-beam-driven deposition of powdered material to rapidly fabricate objects.
Processing of materials into desired shapes and assemblies has traditionally been approached by the combined use of rough fabrication techniques (e.g., casting, rolling, forging, extrusion, and stamping) and finish fabrication techniques (e.g., machining, welding, soldering, polishing). To produce a complex assembly in final, usable form (xe2x80x9cnet shapexe2x80x9d), a condition which requires not only the proper materials formed in the proper shapes, but also having the proper combination of metallurgical properties (e.g., various heat treatments, work hardening, complex microstructure), typically requires considerable investment in time, tools, and effort.
Because of the factors referred to above, there has been interest for some time in techniques which would allow part or all of the conventional materials fabrication procedures to be replaced by additive techniques. In contrast to most conventional materials fabrication techniques, which focus on the precision removal of material, additive techniques are based on the (usually computer-controlled) precision addition of material. Additive techniques also offer special advantages, such as seamless construction of complex configurations which, using conventional manufacturing techniques, would have to be assembled from a plurality of component parts. For the purposes of this specification and the appended claims, the term xe2x80x98pluralityxe2x80x99 consistently is taken to mean xe2x80x98two or morexe2x80x99.
Additive techniques are particularly appropriate for automation, for example the creation of objects based on a computer model and fabrication of small numbers of essentially identical copies. They offer the potential for providing new capabilities in rapid prototyping, rapid manufacture, and rapid fabrication of forms and dies used in mass production.
What portion of conventional manufacturing techniques can be replaced by additive techniques depends, among other factors, on the range of materials available to be deposited using additive techniques, the feature size and the surface finish achievable using additive techniques, and the rate at which material can be added. The ultimate goal is to develop additive techniques capable of fabricating complex precision net-shape components ready for use. In many cases, some degree of finishing will actually be required; such a product is termed near-net shape. Of course, crude billets and approximate forms (preforms) requiring considerable conventional materials fabrication processing for the ultimate application can also be made using additive techniques.
Various approaches exist to use localized energy provided by a laser to deposit a material onto a growth surface. A common example is laser spray techniques, in which a powder is sprayed through a laser beam. The exposure to the laser beam is sufficient to melt the powder, which then cools, solidifies, and adheres to a growth surface, which can be heated, but which remains solid in either case. A related technique is selective laser sintering, in which a thin layer of powder is laid on a growth surface. A laser then traces the solid cross-section of a desired part in the powder, thereby heating the powder and acting to sinter the powder to the growth surface. These techniques are suited to reproduce a shape, but are quite inflexible concerning the material properties of the final piece. In particular, making a component whose material is 100% dense (essentially void-free) is virtually impossible using these techniques.
The above techniques, at least in principle, can fabricate objects from a wide range of materials. Another class of laser-driven additive fabrication techniques, known as stereolithography, essentially function only for photopolymerizable polymers. These techniques typically focus an ultraviolet laser on a growth surface coated with a thin layer of a fluid monomer (or pre-polymer). Where the laser hits, a solid polymer is produced, and a three-dimensional object can be built up by addition of multiple layers. This class of techniques is limited to fabricating plastic objects. Although such objects can at times be used as molds to aid in the production of more general components, such use often fails to eliminate the bulk of the expensive conventional fabrication processing.
Non-laser based additive techniques for rapid fabrication exist. Examples include ink-jet printing techniques, in which the ink is replaced by a thermoplastic material which sets when it hits the growth surface, and closely related bonding techniques, which use a series of nozzles to draw a pattern of a binder on a thin powder layer atop a growth surface. Where the binder hits, a solid material is formed which adheres to the growth surface. As is usual in additive techniques, the thickness of the desired object is built up by xe2x80x9cprintingxe2x80x9d multiple layers atop one another. These techniques are generally limited to fabrication of molds with a large organic component.
The limitations and difficulties in using these additive fabrication techniques exposes the need for a new class of additive fabrication techniques capable of fabricating net shape or near-net shape objects of a wide variety of materials such that the objects exhibit essentially theoretical material density (i.e., the material making up the objects is essentially free of voids).
An interesting approach to laser-driven additive fabrication technique appears in U.S. Pat. No. 4,323,756 (Brown et al.), which teaches building up a solid object by directing an energy beam (restricted to laser beams and electron beams) onto the surface of a substrate, thereby melting the surface of the substrate to a shallow depth.
A feed powder (or a feed wire) is introduced into the molten region from a single off-axis position. When the feed material melts in the molten region, the volume of the molten region increases. That volume, however, is roughly determined by the laser energy input, the melting point of the growth surface, and the thermal characteristics of the material making up the growth surface.
Accordingly, when feed material is added to the molten region, it forces some of the material making up the molten region to freeze onto the growth surface, forming thereon a layer of new material. As the laser is scanned across the growth surface, a solid layer is produced, and a specific object geometry can be built up by growing multiple layers in the manner described above. This technique is adaptable to the deposition of many organic materials and plastics, but also to metals, glasses, and certain other classes of inorganic materials. As the layers being grown solidify from a molten region, essentially void-free material nearly always results.
Brown et al. describe many of the features sought to fill the need for new and more capable additive fabrication techniques. However, their system has difficulties in operation which greatly limit its practical use. Perhaps foremost among these is the tendency of such techniques to exhibit low-frequency fluctuations in the layer growth rate. When such fluctuations occur, individual layers have significantly non-uniform thickness, leading to a rippled surface, poor dimensional tolerances, and inhomogeneous material properties. The layer thickness has also been found to depend on the direction of translation of the laser beam across the surface relative to the orientation of the off-axis powder feed source. This also leads to an unsatisfactory process for rapid and routine application to fabrication of complex components.
There is thus a need for an additive laser-driven material fabrication technique which will share the virtues of the teachings of U.S. Pat. No. 4,323,756, but which will avoid the shortcomings thereof, thereby allowing practical fabrication of complex objects.
The present invention provides a system for fabricating objects whereby an energy beam directed onto a growth surface creates a melt-pool thereon. A converging stream of powdered material is injected into the melt-pool, and melts or dissolves therein, thus adding to the volume of the melt-pool. The volume of the melt-pool being roughly fixed by thermal balance dynamics, this injection forces some of the contents of the melt-pool to freeze onto the growth surface, thereby forming a new layer. This process is carried out over appropriate regions of the growth surface to form thereon a cross-section of the desired object. Multiple layers can be formed in a sandwich structure to build up a complex three-dimensional structure.